A capacitor is a device used in electronic circuits to store electrical charge. Electrical charge Q, is measured in coulombs and one electron has a charge of about 1.6.times.10.sup.-19 coulombs. Typically, the electrical charge is stored on the surfaces of plates separated by a dielectric. The plates are generally layered and may be planar or wound, as for example in a spiral roll. The charge creates an electrostatic field which exists between the plates and therefore creates a potential difference, or voltage V, to exist between the plates.
Capacitance C, is measured in farads and a farad is defined as one coulomb per volt. In general, the capacitance of a device is determined by dividing the charge stored on the plates by the voltage the charge creates across the plates. By increasing the capacitance, a greater charge can be stored per unit volt.
Generally, capacitance can be increased in two ways; by increasing the area of the plates, and by reducing the separation distance between the plates. In an electrolytic capacitor, capacitance is achieved on the anode (+) plate by electrolytically forming a thin layer of dielectric oxide on the surface and immersing it in an electrolyte solution which functions as the negative (-) plate.
The energy, in joules, stored in a capacitor equals 1/2CV.sup.2. In some applications, it is desired to maximize the energy density of a capacitor. One such application is in the biomedical arts, and especially in implantable devices such as medical defibrillators.
Defibrillators must be capable of supplying an intense burst of energy to the heart in a very short time. The battery power supply of a typical implantable defibrillator is incapable of producing this energy alone. Therefore, the battery is used to charge an electrolytic capacitor which is then used to deliver the energy to the heart. For obvious reasons, it is important to minimize the size of the capacitor which is usually the largest component in the defibrillator circuit. It is thus advantageous to use a capacitor having as high an energy density as possible. Flat capacitor configurations, although providing thinness, are complex to manufacture and do not generally provide the overall packaging density achievable in wound configurations. Machine wound configurations are generally easier to produce but are subject to packaging inefficiencies.
One example of an electrolytic capacitor intended for use in a defibrillator is disclosed in U.S. Pat. No. 5,584,890. The multiple anode capacitor disclosed in the patent achieves increased capacitance and energy density over other flat capacitor designs. The method of lead placement and welding together of anode plates however, is not applicable to wound capacitors with long anodes.
What is needed then, is a wound multiple anode capacitor which provides maximum energy density and is acceptably thin.
One object of the invention is to provide a multiple anode capacitor which provides maximum energy density.
Another object of the invention is to provide an extremely high energy density capacitor especially suited for use in implantable medical devices such as defibrillators.
These and other objects of the invention will become apparent from the following recitation of the invention.